The present invention relates to automatic hue control circuitry for a color television receiver in which a VIR reference signal containing unique hue information is relied upon for automatically establishing hue corrections in a displayed image.
The invention relates to control circuits of the general type set forth in the U.S. Pat. No. 3,950,780 issued to Harry T. Freestone and assigned to the assignee of the present invention. The Freestone patent relies on the proposition, still true today, that when the phase of the chrominance reference portion of the VIR signal is the phase of one color difference signal then at the output of the color difference detector of the television receiver the other color difference signal in quadrature therewith is zero. Thus, if the phase of the chrominance reference is minus (B-Y) then the R-Y color difference signal detector output should be zero, i.e., a null which is the same output that the detector has when no chroma information is transmitted. This unique characteristic permits the use of a feedback control circuit to set the hue of the receiver by automatically adjusting the tint control until the R-Y output of the color difference signal detector is zero during the presence of the VIR signal.
Employing this proposition, the Freestone patent teaches how the problem of drift may be overcome in the process of correcting the hue setting of the receiver in accordance with the VIR signal. Freestone achieves this in one embodiment by double interrogation of the VIR signal; a first interrogation takes place during the chroma reference portion of the VIR signal and a second interrogation takes place during a no chroma portion of the signal. Any difference between these two interrogations, as noted at the R-Y output of the color difference detector is used as a control signal to adjust the hue of the receiver. Any drift in the color difference signal detector output or drift in the reference loop is thereby overcome by the periodic updating of the no chroma reference for the control loop.
The present invention contemplates an improvement over the Freestone invention by single interrogation of the VIR signal in a manner which nevertheless overcomes the drift problems identified by Freestone. By employing a single interrogation, the present invention eliminates the necessity of developing two separate VIR timing pulses, one to identify the chrominance reference portion and another to identify the no chroma portion of the VIR signal required by a double interrogation system.
It is accordingly an object of the present invention to provide an improved automatic hue control circuit for a color television receiver which is essentially insensitive to drift problems.
Another object of the present invention is to employ an automatic hue control circuit utilizing AC coupling to enable single interrogation of the VIR signal.
The automatic hue control system of the present invention comprises a circuit enabled by a single VIR timing pulse for establishing an error signal representing any variation in level of a particular color difference signal, the R-Y signal in the preferred embodiment, from the time when the chrominance reference portion of the VIR signal is present to the time when no chroma content is present. No chroma content time occurs essentially (with the exception of burst) during the remainder of line 19 including the remainder of the VIR signal. This period of no chroma input establishes a quiescent null output from the receiver's color difference detector. The output of the receiver's R-Y color difference signal detector is AC coupled to an AC amplifier so as to be responsive only to the variations in the output of the color difference detector during transition to the chrominance reference portion of the VIR signal from the quiescent no-chroma condition, or to the quiescent condition from the chroma reference condition. Also, because of AC coupling throughout the hue correcting loop, DC drift in the quiescent zero reference level of the detector or DC drift in the power supply to the control loop does not affect the correcting action of the control loop. When a variation from the null condition does appear during the chrominance reference portion of the VIR signal, this variation is amplified by the AC amplifier to correct the phase of the local subcarrier oscillator to reduce the one color difference signal to a null condition. A sample and hold gating circuit, activated by the single VIR timing pulse enables the output of the AC amplifier to vary the operation of the hue control circuit only during a select portion of the VIR signal including at least a part of the chrominance reference portion of the VIR signal.
In accordance with another aspect of the invention, the corrective or control signal output of the AC amplifier is coupled to the subcarrier phase control circuit by means of a preference control circuit. The preference control circuit enables the television receiver to create a condition of offset to the control signal so that the personal hue taste of the viewer is accurately reproduced.
A preference control circuit capable of satisfying this object is described in copending application Ser. No. 663,483 filed Mar. 3, 1976 by Brown and Freestone, now continuation application Ser. No. 821,062, entitled Automatic Hue Control with Preference Capability and assigned to the assignee of the present application. Somewhat different in concept from the Brown and Freestone preference control circuit, the preference control circuit of the present invention operates upon the principle of establishing a fixed control range about the control voltage so that regardless of the amplitude of the control voltage the preference will ride on top thereof and represent a fixed hue change.
It is accordingly another object of the present invention to provide precise viewer preference control for VIR hue correction.